In the manufacture of translucent containers such as clear or colored glass bottles and jars, various types of checks or other commercial variations can occur in the walls of the containers. For example, glass particles, knots and lumps, and opaque stones or other occlusions can occur in the container bottom (i.e., the container base and heel) that affect commercial desirability of the container. On the other hand, baffle scars can occur in the container bottom without affecting commercial acceptability of the containers, and code rings can be molded into the container bottom for optically associating the container with its mold of origin. It is therefore desirable on the one hand to provide an inspection technique for detecting and quantifying potentially undesirable variations such as stones, glass particles, knots and lumps, while at the same time substantially ignoring mold code rings and variations such as baffle scars that do not affect commercial acceptability of the container.
It has heretofore been proposed to employ electro-optical inspection systems for detecting commercial variations that affect optical properties of the container. The basic principle is that a light source is positioned on one side of the container and a camera is positioned on the other. The light source may be configured to have an intensity that varies across one dimension of the source. Light rays normally travel from the source straight through the container wall and are then focused onto the camera, and are viewed by the camera at a given intensity. However, a refractive commercial variation bends the light ray as it travels through the container wall, so that the image projected onto the camera is of a different area of the light source. If such different area has a different intensity than the area normally imaged onto the camera, the camera can detect the refractive site. An opaque site such as a stone blocks transmission of light through the container wall, and can therefore be detected as a dark spot against a light background.
U.S. Pat. No. 5,243,400, assigned to the assignee hereof, discloses a technique for varying the effective intensity of the light source across the light source. A light control film is positioned adjacent to the light source between the light source and the container, and comprises a plurality of parallel slats spaced from each other so as to limit the angle from which the image of the light source can be viewed by the camera. Commercial variations are detected as a function of variations in light intensity received at the camera. Any refractive variation in the container wall refracts or bends the line of sight of the camera at an angle to the camera axis. As this angle increases, the slats progressively obstruct the light source, until the camera can no longer see the light source at the critical viewing angle of the slats.